This invention was made with United States Government support under Award 70NANB5H1070, entitled "High Information Content Display Technology", awarded by the Department of Commerce through its National Institute of Science and Technology (NIST).
This invention relates to color projection display systems employing one or more electro-optic light modulators, and more particularly relates to such a system which employs a white light illumination source, and dichroic filters to separate the white light into primary color components, and to recombine these components.
Most color projection display systems employing one or more electro-optic light modulators, also employ a single white light illumination source, a first set of dichroic filters arranged to separate the white light from the source into primary (red, blue and green) color components, and a second set of dichroic filters to recombine the components after modulation.
The design of such systems dictates that the dichroic filters be inclined at an angle to the paths of the light beams, usually 45 degrees. These inclined surfaces in the optical paths break what would otherwise be a symmetrical arrangement of light rays with respect to the top and bottom (or left and right, depending on the layout) of modulator panel(s), and corresponding asymmetry of light rays in the projected image.
This asymmetry, and the well-known sensitivity of the transmission characteristics of such filters to the incident angles of the light rays striking their surfaces, results in a variation of cut-off wavelength of the filters, and a consequent variation in hue of the primary colors across the display image, the direction of variation (width or height) depending on the direction of tilt of the dichroic filters in the lightpath.
In co-pending U.S. patent application Ser. No. 09/100,829 filed Jun. 19, 1998, now U.S. Pat. No. 5,999,321, the specification of which is incorporated herein by reference, dichroic filters with reduced angle sensitivity and sharper cut-off wavelengths are described. While use of such improved filters in a color projection display system reduces hue variation, appreciable hue variation can still occur.